The present invention relates to a gel-forming composition based on (a) water, (b) a water-soluble cellulose mixed ether having at least one phosphorus-containing ether substituent and (c) a salt having a crosslinking action. Furthermore, the invention comprises a process for the preparation of a gel from this composition, a process for the reversible reversal of the gel thus produced and the use of the gel-forming composition in the secondary production of petroleum.
Commercially available types of water-soluble cellulose ethers exhibit Newtonian or non-Newtonian flow behavior in aqueous solution, as a function, inter alia, of their average viscosity values. Without special modifications they have, in general, no or only a very small thixotropic flow or gel-formation tendency within the concentration range of up to 2%, which is very common in practice and within which viscosities of up to more than 10.sup.5 mPas (measured by the Hoppler method at 20.degree. C. in a 2% strength aqueous solution) can be achieved. It may be assumed that in these aqueous solutions the dissolved polymeric cellulose ether molecules form only weak hydrogen bonds among one another, the bond strength of which is insufficient to impart a gel-like structure to the aqueous system. Depending on the type of cellulose ether, the addition of certain modifying additives to the aqueous solution can initiate chemical crosslinking reactions producing stable chemical bonds between individual or several cellulose ether polymer chains, which bonds lead to the formation of a stable gel structure throughout the entire volume of the aqueous system. If the concentration in the aqueous system of the cellulose ether and/or of the crosslinking additive is kept at a low value, the result is frequently not the formation of a gel but only an increase in the viscosity. The latter phenomenon is often useful in those cases in which the cellulose ether is intended to have only a thickening function, since it is then possible in these areas of application to employ amounts of cellulose ether which are low compared with unmodified cellulose ether types.
In particular the following abbreviations and parameters are customary in the nomenclature and characterization of cellulose ethers and will also be used in the text below: C=Cellulose, Alk=Alkyl, M=Methyl, E=Ethyl, HAlk=Hydroxyalkyl, HE=Hydroxyethyl, HP=Hydroxypropyl, HB=Hydroxybutyl, CAlk=Carboxyalkyl, CM=Carboxymethyl, NaCM=Sodium (Na) carboxymethyl, CE=Carboxyethyl, SAlk=Sulfonoalkyl, SE=Sulfonoethyl, PAlk=Phosphonoalkyl, PM=Phosphonomethyl, PP=3-Phosponopropyl, (MPM)=Methylphosphino-methyl, (MPP)=3-Methylphosphino-propyl, NaCMHE=Sodium (Na) carboxymethylhydroxyethyl, MPM=Methyl-phosphonomethyl; DS=degree of substitution, that is to say the average number of substituted OH groups per anhydro-D--glucose unit--for cellulose it is within the range of 0.0 to 3.0; MS=molar degree of substitution, that is to say the average number of moles of the substituting reagent which are bonded ether-like per mole of anhydro-D--glucose unit--for cellulose it can also be greater than 3.0 and is normally used instead of DS to characterize those substituents on a cellulose ether which can be the result of a multiple substitution at an OH group (in the case of hydroxyalkyl groups); DS.sub.PM =degree of substitution of a cellulose ether in respect of phosphonomethyl substituents; MS.sub.HE =molar degree of substitution of a cellulose ether in respect of hydroxyethyl substituents.
Cellulose ethers which can be influenced in the direction of a strong viscosity increase or can be gelled by additives and which include in particular those which have anionic substituents, such as carboxymethyl groups, in the molecule are frequently used in the secondary production of petroleum. Secondary production is here understood as meaning the procedures in the recovery of petroleum which are started after the primary production, which is caused by natural or additionally aided natural forces (such as pressure due to natural gas or breaking up of underground formations). Secondary recovery procedures are becoming increasingly more interesting and important for economic and ecological reasons owing to increasing revenues from crude oil. This secondary production of crude oil from underground formations is caused, for example, by liquids which are introduced into the formation via additional wells (injections) in order to displace the petroleum from the formation toward the actual production well. For example, a water-soluble hydrocolloid, such as natural polymeric products (for example xanthane resin), a cellulose ether or an acrylic polymer, can be added to these liquids in order to increase their viscosity and hence to be able to displace the petroleum in an improved and more effective manner. In order to be employed for this purpose, hydrocolloids have to meet the following conditions, among others: They should increase the viscosities of the aqueous liquids to a very considerable extent, even when low quantities of hydrocolloid are added, or produce a stable mobile gel. They should also be soluble in salt solutions and ideally not coagulate in them and hence become ineffective, since underground formations frequently contain soluble salts or the liquids to be injected are already salt-containing. Also, the present or "in situ" produced viscosity, or, respectively, the gel produced should be stable over relatively long time periods at up to relatively high temperatures and at high shear values; that is to say under conditions as encountered in underground formations.
In the text which follows, terms from the field of colloid chemistry are used as they are defined in Roempps Chemie-Lexikon (Dictionary of Chemistry), Franckh'sche Verlagsbuchhandlung--Stuttgart, 7th Edition, 1976, entries "Gele (gels)" and "Kolloidchemie (colloid chemistry)," page 1244 and pages 1821 to 1827. A gel is in particular understood as meaning a state in which there are molecules of a liquid (normally water molecules) arranged among the solid, colloidally divided cellulose ethers. Other names for this state are lyogel (hydrogel) or jelly. It is known that the stability of such gels is a function of, inter alia, the pH value of the system or the presence of foreign ions.
The state of the art concerning the preparation of stable cellulose ether gels and/or their application has been disclosed, for example, in the printed publications which follow:
Ullmanns Encyklopadie der technischen Chemie (Ullmann's Encyclopedia of Industrial Chemistry), Verlag Chemie--Weinheim, 4th Edition--1975, Volume 9, entry "Celluloseaether (cellulose ethers)," pages 192 et seq. states (page 196) that salts have a considerable influence on the gel point of cellulose ether solutions. Multivalent cations, such as Al.sup.3+ ions or Cu.sup.2+ ions, are said to be particularly able to effect up to quantitative precipitation of NaCMC from their aqueous solutions (pages 197 and 211). Solutions of typical water-soluble cellulose ethers are in general non-Newtonian, that is to say their viscosity depends on the shear force or the shear rate (page 199), the result of which is that, in particular in the case of high viscosity cellulose ether types, differing viscosity values are produced as a function of the method of measurement. Only in the case of very low viscosity cellulose ether types is Newtonian flow also found.
German Auslegeschrift, No. 1,147,751 discloses a process for the preparation of gels of those cellulose ethers which carry carboxymethyl groups as substituents, in which process aqueous solutions of these cellulose ethers are reacted with aluminum alcoholates of saturated aliphatic alcohols. Gel formation is said to be encouraged by weakly acid pH values (for example of 4 to 5). If the amount of aluminum alcoholate added to too low, merely an increase in the viscosity is said to occur, a quantity of 12% (relative to dry cellulose ether) being considered adequate for gel formation. The resulting gels are suitable for the preparation of films, compositions for dental impressions or of engobes. In the discussion on the state of the art, mention is also made of a reaction of cellulose ethers with Cr.sup.3+ ions resulting in the formation of a gel. Cellulose ethers mentioned as suitable for the process are NaCMC, CMC and NaCMHEC.
Pure or mixed cellulose ethers having 2,3-dihydroxypropyl groups as substituents, in accordance with German Offenlegungsschrift No. 2,415,154 or German Offenlegungsschrift No. 2,415,155 (=U.S. Pat. No. 4,001,210), can be reacted with compounds which provide borate ions, such as boric acid, borates or readily hydrolyzable boric acid esters, to give highly viscous products or stable gels. The reaction with borate ions is carried out either in an alkaline reaction medium or by the addition of borates to aqueous solutions of the cellulose ethers.
In a process for the preparation of a thickened, aqueous salt solution according to German Offenlegungsschrift No. 2,639,620 (=U.S. Pat. No. 4,035,195) CMHEC types having a DS.sub.CM of 0.2 to 0.6 and having an MS.sub.HE of 1.5 to 3.0 are crosslinked in an aqueous solution with multivalent metal cations. The concentration of cellulose mixed ether in the solution is 0.025 to 1%, and the molar ratio of the metal ions to carboxyl groups of the cellulose ether is 0.02 to 1. Suitable metal cations are said to be Fe.sup.3+, Al.sup.3+, Cr.sup.3+ and Zr.sup.4+, with cellulose ethers cross-linked with these being able to yield high viscosity solutions and also stable gels. The secondary production of petroleum is mentioned as a field of application, whereby salt solutions present underground have to be thickened.
German Offenlegungsschrift No. 2,928,247 (=U.S. Pat. No. 4,183,765) describes a process for increasing the viscosity of an aqueous HAlkC solution, in which process the aqueous solution contains at least 0.075% by weight of the cellulose ether, and 0.4 to 75% (relative to the weight of the cellulose ether) of benzoquinone are added to this solution, and this solution then has a pH value of over 6.4. It is said to be possible to influence the viscosity not only of HEC or HPC but also of their mixed ethers (for example HECMC).
The preparation of shaped products (fibers, tapes or films) based on alkali-, water- and acid-insoluble CAlkC is known from U.S. Pat. No. 2,420,949, and although in principle the free acid form of the cellulose ethers is present, at least some of the carboxyl groups are crosslinked by Zr.sup.4' ions or ZrO.sup.2+ ions and the content of Zr ions (calculated as ZrO.sub.2) in the crosslinked cellulose ether is 3 to 9%.
Pure or mixed cellulose ethers containing at least 1.4 dihydroxypropyl groups per anhydro-D-glucose unit in accordance with U.S. Pat. No. 4,096,326, are water-soluble and thermoplastic and can be treated with 0.05 to 10 parts by weight of borate ions or antimonate ions per part by weight of cellulose ether in solution. The resulting cellulose ether borate or cellulose ether antimonate complexes have, in aqueous solution, a considerably increased viscosity compared to untreated cellulose ethers, which viscosity persists also in salt solutions or is even higher therein. Products thus modified are intended to be used, for example, in the secondary production of petroleum.
In a process for the gel-forming setting of drilling muds, described in German Pat. No. 2,109,823, aqueous solutions of metal salts of multivalent cations, such as Al.sup.3+, Fe.sup.3+, Cr.sup.3+, of Cu.sup.2+, which salts have an acid reaction, are added "in situ" to systems containing CMC (as a prototype of a polymer which contains carboxymethyl groups). Drilling mud solutions themselves are in general rendered neutral or slightly alkaline and contain 0.2 to 4% by weight of the polymer which contains carboxymethyl groups. Quantity ratios indicated in the examples for the salt added are 0.03 to 0.2 parts by weight of salt per part by weight of cellulose ether.
German Offenlegungsschrift No. 2,544,777 (=British Pat. No. 1,503,897) describes a fragrance carrier based on CMC gels crosslinked with at least trivalent metal ions. The gels contain 0.5 to 10% by weight of CMC and 0.2 to 5% by weight of metal salts having Al.sup.3+, Fe.sup.3+ or Cr.sup.3+ ions.
European Published Application No. 0,007,012 and European Published Application No. 0,007,013 disclose gelling compositions for the secondary production of petroleum which contain, in addition to water and 0.1 to 3.0% of a thickener, such as a cellulose ether (for example CMC, CEC, CMHEC, HEC, HPC, MHPC, MC, EC, PC, ECMC, MEC or HPMC), also about 0.001 to about 1% of an aldehyde and/or of a phenolic component. These gelling compositions may also contain, if appropriate, additionally 0.4 to 35% of an acid.
The process for consolidation of wells which is described in U.S. Pat. No. 2,439,833 is carried out either by introducing an aqueous NaCMC solution having an adequate quantity of certain salts into the porous underground formations of a well or by additionally introducing an aqueous solution of these salts, namely, FeSO.sub.4, FeCl.sub.3, Ba(NO.sub.3).sub.2, SnCl.sub.2, Pb(CH.sub.3 COO).sub.2, or AL.sub.2 (SO.sub.4).sub.3 into the well, in order to effect the consolidation (for example by means of gel formation). This consolidation can be reversed by the addition of water-soluble hydroxides, such as NaOH.
According to U.S. Pat. No. 3,804,174, a consolidation medium for wells can contain, in addition to cement and water, a reaction product formed from a water-soluble cellulose ether and a multivalent metal ion. Cellulose ethers include HEC, CMC and CMHEC and the metal ions include Zr.sup.4+, Pb.sup.2+, Cr.sup.3+, Fe.sup.3+, Hf.sup.4+, La.sup.3+ and Y.sup.3+, and in particular ZrO.sup.2+.
In U.S. Pat. No. 3,971,440, the disclosed process for improving the secondary production of oil by means of aqueous gels also employs a polyacrylamide in addition to water, a cellulose ether, a reducible metal ion and a reducing agent for the metal ion.
U.S. Pat. No. 4,018,286 describes a process for the preparation of a temporary consolidation of an underground formation, in which process a composition of (a) a gellable polymer, such as a cellulose ether or an acrylic polymer and (b) a complex formed from an Fe.sup.2+ cation, Fe.sup.3+ cation, Al.sup.3+ cation, Ti.sup.4+ cation, Sn.sup.2+ cation, Ca.sup.2+ cation, Mg.sup.2+ cation or Cr.sup.3+ cation and an anion which is suitable for complexing the cation, such as a tartrate ion or a citrate ion, is maintained at a pH value of 3 to 7 for as long as the consolidation is intended to last, and in which process a reversal of the consolidation is effected by lowering or increasing this pH value.
The process in accordance with U.S. Pat. No. 4,096,074 for the secondary production of petroleum comprises also the addition of an aqueous solution having a thickening action, comprised of a reaction product formed from an organic polyisocyanate and a linear, nonionic polysaccharide ether (for example HEC, HPC, MHPC or HEHPC), the polyisocyanate acting as a crosslinking agent for the polysaccharide ether.
These very numerous publications show that there is obviously no universally applicable means which can be used in all the very diverse fields of application of aqueous gels.
Frequently the gels described are stable only within certain pH ranges and reliquefy on changes in the pH value. In some cases complex compositions and/or expensive organic additives are required to produce a gel. To make possible economical and problem-free application, in particular in the field of the secondary production of petroleum which employs very large quantities, the gel components must be inexpensive and simple to handle. Possible active components have up to now been essentially the polysaccharide derivatives CMC (NaCMC) or CMHEC which contain carboxymethyl groups and inexpensive inorganic salts of multivalent cations, such as Al.sup.3+ and Cr.sup.3+. However, these cellulose ethers form stable gels only within the neutral range, while no gelling effect is achieved with multivalent cations in the alkaline or strongly acid range. Instead, cloudings and precipitations of the cellulose ether occur.